A:What
B:That
C:Which
D:Whatever
在地球的任何地方面北而立,皆左西右东。 ( )
Helen is going to work in an old people's home to help ________ the old people.
某离子晶体中晶体结构最小的重复单元如下图:A为阴离子,在立方体内,B为阳离子,分别在顶点和面心,则该晶体的化学式为( )